This will not work - since you do not only require the certificate but also the corresponding private key (which should never be disclosed).You seem to be seeking for a Principal Propagation mechanism (in a system-to-system communication).Maybe OAuth...
Gregor has already provided the correct answer. SAP Cloud Identity Services are a group of services, including IAS (Identity Authentication Service) and IPS (Identity Provisioning Service). Self-registration is supported by IAS and using IPS will ens...
For on-Premise ABAP systems there are several Identity Management solutions (IdM) which are using the RFC-based BAPI_USER_* function modules. See: SAP Note 3003462
Have a kind look at Manually Testing SAP BTP ABAP Environment APIs with Postman using OAuth 2.0 Authorization Code GrantPKCE with OAuth 2.0 Authorization Code Grant is supported as of release 2308. Regards, Wolfgang